For hormones such as cortisol to have an effect, they must bind (attach themselves) to receptors that are located throughout the body. It is like the hormones are car keys, and the receptors are the ignition. Like the key fits in the ignition, turns and starts the car’s engine, the hormone binds to the receptor, activates it and starts chemical reactions throughout the body. And just like only your car key can fit in the ignition of your car to start it, only certain hormones can fit with, or bind to certain receptors and cause an effect. There are two types of receptors that cortisol can bind to and activate, mineralocorticoid receptors (MR’s) and glucocorticoid receptors (GR’s).

Different things happen depending on which receptor cortisol activates, and whether it fully activates the receptor, or partially activates it. If we continue with the car analogy, partial activation of a receptor is like the way you can use your car key to turn on only accessories like the radio and power windows, and full activation would be the same as turning your key to start the engine. Just like your car does different things depending on how far the key is turned, your memory does different things depending on how fully cortisol binds to and activates the MR’s and GR’s .

This is the physiological reason why stress hormones sometimes make your memory of a stressful or traumatic event very clear, and sometimes make it fuzzy, fragmented or apparently non-existent. If cortisol fully activates the MR’s and at the same time only partially activates the GR’s, then your memory will be improved. But if the GR’s are also fully activated, then your ability to remember something is impaired (Abercrombie et al., 2003). If there’s lots of cortisol, lots of GR’s become fully activated and this makes it harder to remember stuff. But the opposite is true as well; if there’s not enough cortisol to activate enough receptors, that makes it harder to remember something (Rimmele et al., 2010). Let’s change the analogy and imagine your memory is like a balloon and cortisol is the air we’re putting in it. Without enough air, the balloon isn’t anything but a piece of rubber; it’s empty. The balloon gets bigger the more air that goes in, but there’s a limit to how much air it can take before it pops. Too much air, and the balloon is just bits of rubber. Too much cortisol, and you just have bits and pieces instead of a whole memory.

The way cortisol enhances or impairs memories becomes complicated for many reasons. The receptors themselves are really important, because it’s easier for cortisol to bind to MR’s than GR’s (McEwen & Sapolsky, 1995; Reul & De Kloet, 1985). Our body always tries to conserve energy when it can, so cortisol binds with MR’s first. Once there are no more MR’s left to activate, cortisol starts activating GR’s. This is why it takes more cortisol to impair memories than it does to enhance them. The GR’s aren’t activated until after the MR’s, and if cortisol levels stop rising when the GR’s are only partially activated, then we get enhanced memories. This also explains why very low levels of cortisol also impair memory; there needs to be enough cortisol to activate the MR’s before we can remember something well (Rimmele, et al., 2013).

Then there’s the difference between chronic (ongoing, long-term) and acute (short-lived) stress. A single stressful or traumatic experience can cause cortisol levels to rise temporarily while the event is happening, but ongoing stress or trauma can cause cortisol levels to drop and stay at levels lower than normal (Meewisse et al., 2007; Rimmele et al., 2013). Your body isn’t designed to operate with constantly high levels of stress hormones, so if cortisol levels get too high for too long, the systems responsible for hormones regulate themselves by dropping cortisol production. This means that ongoing trauma or stress can make it hard to remember stuff because you don’t have enough cortisol, and a single stressful or traumatic event can be hard to remember because you have too much.

Sometimes it’s not even the type, nature, or severity of the stressful or traumatic event that influences how much cortisol is produced, and how your memory is affected. For example, everyone’s cortisol levels vary naturally throughout the day, regardless of what’s going on around you (Meewisse et al., 2007). This means that your memory or any event, stressful or not, is potentially influenced by what time the event occurred. About twenty minutes after waking, most people naturally have cortisol levels high enough to partially activate GR’s, but not enough to do the same in the early evening (Meewisse et al., 2007). So it’s possible that if you experience enough stress to further raise your cortisol levels in the morning, your memory of the stressful event might be impaired because the increased cortisol binds to and fully activates GR’s that are already partially activated by your natural morning cortisol levels. But if the stressful event happened in the evening when you have low natural cortisol and therefore no activation of the GR’s, the rise in cortisol caused by stress would only partially activate the GR’s, which means your memory of the event is likely to be improved. The time of day is only one example of many different things that can influence how cortisol effects your memory and other parts of your body. The way memory works is complicated, and when it comes to trauma, the way we remember the events doesn’t necessarily say a whole lot about how that trauma impacted us in other areas, or our ability to recover and heal.

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